Viscosity or viscous properties corresponding to a usage method or usage objective are generally imparted to water-based emulsions, or water-based paints and pressure-sensitive adhesives using such emulsions, and these are normally imparted by viscosity modifiers. Natural viscosity modifiers such as carboxymethyl cellulose and hydroxyethyl cellulose, alkali-thickening viscosity modifiers which increase viscosity with an alkali, such as polyacrylic acid or copolymers including polyacrylic acid, and urethane-type viscosity modifiers such as urethane-modified polyethers are known, for example, and in recent years, demand has grown for urethane-type viscosity modifiers that have good water resistance and are not affected by pH.
The advantage of urethane-type viscosity modifiers is that they come in various structures and can be selected according to the application (see, for example, Patent Documents 1 and 2). Meanwhile, some urethane-type viscosity modifiers have a high viscosity and are difficult to handle, and some have poor miscibility with water-based emulsions. As a result, a long time is often required to uniformly mix them with emulsions. Another problem is that the product stability (referred to hereinbelow as “product stability”) of emulsions to which a viscosity modifier has been added is poor; that is, the viscosities of some emulsions change significantly after a viscosity modifier has been mixed therewith. Accordingly, methods using specific solvents for resolving such problems are known (see, for example, Patent Documents 3 and 4). Such methods make it possible to reduce the viscosity of the urethane-type viscosity modifiers and improve compatibility thereof with emulsions.